Method and device for extending the dynamic range in measurements c arried out with a directional coupler

ABSTRACT

For extending the dynamic range in measurements carried out with a directional coupler, such as antenna measurements, a signal is supplied to the object under analysis, such as an antenna, through a first port of the measuring directional coupler, part of the signal leaking as a leak signal through a second port of the measuring directional coupler directly towards a measuring device. The signal returning from the object under analysis is measured by the measuring device. To enable the measuring of signal levels lower than before, spurious signals, such as the leak signal, summing to the signal to be measured are eliminated by supplying a phased compensation signal through a second directional coupler to the measuring line.

BACKGROUND OF THE INVENTION

The invention relates to a method for extending the dynamic range inmeasurements carried out with a directional coupler, such as antennameasurements. In the method, a signal is supplied to the object underanalysis, such as an antenna, through a first port of the measuringdirectional coupler, part of the signal leaking as a leak signal througha second port of the measuring directional coupler directly towardsmeasuring means, and the signal returning from the object under analysisis measured by the measuring means. The invention also relates to adevice for carrying out the method. The device comprises (i) transmittermeans for generating an electric measurement signal, (ii) a measuringdirectional coupler, to which the transmitter means are coupled forcoupling the measurement signal to the object to be measured, and (iii)measuring means for measuring the signal returning from the object underanalysis. In particular, the invention can be applied to measuring thestanding-wave ratio (SWR) of an antenna in order to observe thecondition of the antenna.

In measurements such as those which are mentioned above the situation isthe following: the signal reflected back from the object to be measured,such as an antenna, is, when summing to the signal leaking from thedirectional coupler straight in the direction of the measuring means, ata random phase, e.g. depending on variations in the electric length ofthe antenna cable. The magnitude of the error caused by summation isthus dependent on the phase difference between the returning signal andthe leaking signal, which varies, e.g., as a function of the electriclength of the antenna cable. Known measuring solutions aim at the bestpossible directivity of the directional coupler supplying themeasurement signal to the antenna, in order to render as small aspossible the spurious signal leaking from the directional couplerstraight in the direction of the measuring means and summing to theactual signal to be measured. The effect of other spurious signals suchas those reflected from the coupling points has been diminished bycalibration and by means of modification tables and correction factors.The obtained directivity of the directional coupler is, however, rarelymore than 30 dB; in general the directivity is in the range of 20 . . .30 dB, as the directional coupler is to be realised on the printedcircuit board as a micro strip structure, which isproduction-technically advantageous, due to easy reproducibility. Owingto the limited directivity, the measuring of low signal levels, such asthe signal reflected back from an antenna demonstrating a minorreflection loss, becomes difficult when the (useful) signal to bemeasured sums to a spurious signal leaking from the directional coupler.On the other hand, the useful signal to be measured sums also to otherspurious signals passing to the measuring point, such as the signalsreflected from the coupling points. In some cases, these other spurioussignals may be greater in magnitude than the above-mentioned leakingsignal, thus preventing the measuring of low signal levels.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to obviate thedrawbacks described above and to improve the dynamic range ofmeasurements carried out with a directional coupler so that themeasuring of lowlevel signals will be effected more successfully. Thisis achieved with the method according to the invention, which ischaracterised by the fact that spurious signals, such as the leaksignal, summing to the signal to be measured are eliminated by supplyinga phased compensation signal through a second directional coupler to themeasuring line.

The basic concept of the invention is to eliminate the dominatingspurious signals by supplying a compensation wave to the measuring lineby means of the second directional coupler.

The solution according to the invention extends the dynamic range of themeasuring by 10 . . . 25 dB, compared with conventional methods ofmeasuring. Another advantage of the solution provided by the inventionis that it enables even the use of a directional coupler with a poordirectivity, since the spurious wave leaking from the directionalcoupler is eliminated in any case. The circuit is also inexpensive torealise, and the actual measuring is quick to carry out, as the errorcorrection is effected in real time. The possible tuning of thecompensation signal needs to be effected only in the production phase.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the example depicted in the attached FIGS. 1-2 whichdisclose a measuring arrangement according to the invention in measuringthe condition of an antenna.

DETAILED DESCRIPTION

The figure discloses the measurement arrangement according to theinvention when the condition of a receiving antenna 10 of the basestation of a cellular network is to be checked. The condition is checkedby defining the standing-wave ratio describing the electric matching ofthe antenna to the rest of the transmitter--and receiver system bymeasuring the power of the signal reflected back from the antenna withthe measuring means 12 at the end of a measuring line 11 and bycomparing this with the power transmitted to the antenna. Only that partof the measuring arrangement which pertains to the invention will be setout. In whole this kind of measuring arrangement is disclosed in FinnishPatent 80,817. At the front end of the measuring line 11 there isdisposed a measuring directional coupler DC1 whose input port 1 iscoupled to a divider 14, a travelling wave port 3 is coupled through anantenna line 10a to the antenna 10, a port 2 is terminated by resistanceR and a gate 4 is coupled to the measuring line 11. Between themeasuring means 12 and measuring directional coupler DC1 there isdisposed a second directional coupler DC2 whose input port 1' is coupledthrough a delay unit 13 to the divider 14, a travelling wave port 3' iscoupled to the measuring line in the direction of the measuring means12, a port 2' is terminated by resistance R and a port 4' is coupled tothe measuring line 11 in the direction of the antenna 10. Transmittermeans 15 are coupled to the divider 14, supplying an electricmeasurement signal A1 through the divider 14 to the directionalcouplers. The measurement signal A1 is formed as described in theabove-cited Finnish Patent 80,817 from the transmitter signal by mixingit with a transfer oscillator to the desired measuring frequency.

In addition to the measurement signal A1, in the figure signals from andto the directional coupler DC1 are marked with reference signs An (n=2 .. . 4) and signals from and to the directional coupler DC2 withreference signs Bn (n=2 . . . 5). Next to the reference signs tinyarrows are drawn indicating the travelling direction of the signal inquestion.

The measurement signal A2 which is supplied to the antenna 10 throughthe divider 14 and the measuring directional coupler DC1 leaks in themeasuring directional coupler DC1 straight in the direction of themeasuring means 12. This leak signal A4 sums to the signal returningfrom the antenna at a random phase causing an error whose magnitude isdependent on the reciprocal phase of the leak signal and the signalreturning from the antenna. In addition to the leak signal, error iscaused, e.g., by the summing of reflections caused by cables andconnectors in the antenna line to the signal returning from the antenna.

The correction of the error (improving the dynamics) can be divided intotwo situations: (1) a situation in which the leak signal A4 of themeasuring directional coupler DC1 is stronger than the other spuriouswaves and (2) a situation in which the other spurious waves are strongerthan the leak signal A4.

In situation (1), which is more general, the leak wave A4 is eliminatedby supplying using a second directional coupler D2 in the direction ofthe measuring means 12, a compensation signal B5 which has an amplitudewhich is equal to that of the leak signal A4, but in relation theretodelayed in its phase by half a wave length (λ/2) or its oddmultiplicity. The compensation signal is formed of the measurementsignal A1 by branching the measurement signal by means of the divider 14through a delay unit 13 to the second directional coupler DC2. In orderfor the amplitude of the compensation signal to be equal to that of theleak wave, the coupling of the second directional coupler DC2 is equalin magnitude to the sum of the coupling and the directivity of themeasuring directional coupler DC1 (no losses in the measuring line). If,thus, for instance, the directivity of the measuring directional couplerDC1 (D₁ =10 log A3/A4) is 20 dB and the coupling (C₁ =10 log A2/A3) 20dB as well, the coupling of the second directional coupler DC2 (C₂ =10log B3/B5) will be 40 dB. In order for the phase of the compensationsignal to be transferred by half a wave length (or its odd multiplicity)with regard to the leak wave, a delay d1 of the delay unit 13 would beof such magnitude that a delay d2 of the signal between the directionalcouplers DC1 and DC2 deducted from the delay d1 has a magnitude of halfa wave length or its odd multiplicity (d1-d2=nλ/2, n=1, 3, 5 . . . ). Asthe compensation signal B5 eliminates the leak wave A4, the signal to bemeasured is no longer dependent on the electric length of the antennaline (antenna cable); in other words, the summing of the signalreturning from the antenna at a random phase to the leak signal nolonger causes measurement error. Measurement error is, however, stillcaused to some extent by other factors such as cables and connectors.The dynamic range of the measurement can nevertheless be extended by 10. . . 25 dB compared with conventional methods.

In situation (2) the compensation signal must be tuned separately toequal in amplitude the spurious signal in the measuring line and to bein its phase opposite to this spurious signal. The tuning may beaccomplished e.g. by terminating the antenna line with a matchedresistance (test load) and carrying out in time domain in a manner knownper sea measurement (time domain analysis) which gives the amplitude andphase of the reflected spurious signal, in other words, the location ofthe point causing reflection and the magnitude of the reflection. Beforethe measurement, already at the planning stage, all points (reflectionpoints) having an effect on the standing-wave ratio in the antenna lineshould be taken into account by adjusting those points at the measuringfrequency in question at intervals of half a wave length or itsmultiplicity. This enables elimination of all factors causing spurioussignals by means of one additional directional coupler DC2.

Although the invention has been described above with reference to theexample depicted in the attached drawing, it is clear that the inventionis not restricted thereto but can be modified in many ways within thescope of the inventive concept set forth above and in the attachedclaims. In principle in place of the directional couplers there could beother directional means, e.g. a capacitively coupled hybrid. For thisreason the used term `directional coupler` should be understood in alarger sense. Neither is measuring limited solely to antennameasurements, but the principle according to the invention can also beused in other measurements carried out with directional couplers. It isalso possible to change the mutual order of the directional couplers.

I claim:
 1. A method for extending the dynamic range in measurementscarried out with a directional coupler, comprising:(a) supplying asignal in a first direction towards an object which is under analysis,through a first port of a measuring directional coupler, whereupon partof said signal leaks as a leak signal through a second port of saidmeasuring coupler, in a second direction to a measuring line, towards ameasuring device; (b) measuring a returning signal returning from saidobject, using said measuring device; and (c) while conducting steps (a)and (b), eliminating a spurious signal comparable to said leak signalpropagating in said second direction towards said measuring device, bysupplying through a second directional coupler, to said measuring line,a phased compensation signal propagating in said second directiontowards said measuring device with substantially the same amplitude as,but substantially opposite in phase to, said spurious signal.
 2. Themethod of claim 1, wherein step (c) further includes:delaying saidcompensation signal, so that the phase difference between saidcompensation signal and said spurious signal has a magnitude of half awavelength, or an odd integer multiple of a wavelength; and adjustingthe amplitude of said compensation signal to be equal to the amplitudeof the leak signal, by adjusting coupling of said second directionalcoupler to be equal in magnitude to the sum of the coupling anddirectivity of said measuring directional coupler.
 3. The method ofclaim 1, wherein step (c) further includes:determining said spurioussignal occurring in said measuring line, by making a separatemeasurement thereof; and turning said compensation signal based on saidmeasurement.
 4. The method of claim 1, wherein:said object is anantenna.
 5. Apparatus for extending dynamic range in measurementscarried out with a directional coupler, comprising:a transmitter forgenerating an electric measurement signal; a measuring directionalcoupler to which said transmitter is coupled for supplying themeasurement signal towards an object to be measured; measuring equipmentcoupled to a measuring line for measuring a returning signal returningfrom said object; said measuring directional port having a first portfor travelling wave, said first port being coupled to a first directiontowards said object for supplying a measurement signal to said object;said measuring directional port having a second port, said second portbeing coupled to the measuring line to a second direction towards themeasuring equipment; part of the measurement signal leaking in use, as aleak signal through said second port to said measuring line andpropagating towards said measuring equipment; a second directionalcoupler, said second directional coupler being coupled to the measuringline and having a third port for a travelling wave, said third portbeing coupled to said measuring line to a third direction towards themeasuring equipment for supplying a phased compensation signal towardssaid measuring equipment; said compensation signal being ofsubstantially a same amplitude as, but of substantially opposite phaseto, a given spurious signal, summing to said returning signal; and saidtransmitter being operatively coupled to said second directional couplerthrough a delay unit for adjusting the phase of the compensation signal.6. The apparatus of claim 5, wherein:coupling of said second directionalcoupler is equal in magnitude to the sum of directivity and coupling ofsaid measuring directional coupler.